JP-8-338285A (U.S. Pat. No. 5,730,111) discloses a technique in which, to improve air-fuel ratio control accuracy by reducing variations in the air-fuel ratio among cylinders of an internal combustion engine, at the time of performing air-fuel ratio detection by an air-fuel ratio sensor, a cylinder from which an exhaust to be actually detected came is specified and a feedback control of the air-fuel ratio is performed on the specified cylinder.
JP-3-37020B discloses a technique in which an air-fuel ratio of an exhaust collection part is detected using an air-fuel ratio sensor, and in view of a delay until the exhaust of a cylinder reaches the air-fuel ratio sensor, the fuel supply amount of the cylinder is corrected.
It is considered that the response of the air-fuel ratio sensor varies between the case where a rich output is detected and the case where a lean output is detected. Therefore, a sensor output of high response and a sensor output of low response mixedly exist and a problem occurs such that variations among cylinders cannot be eliminated with reliability.
Japanese Patent No. 3217680 (U.S. Pat. No. 5,657,736) discloses a system in which a model describing the behavior of an exhaust system in an internal combustion engine is set. A detection value of one air-fuel ratio sensor mounted in an exhaust collection part (an air-fuel ratio of exhaust gas flowing in the exhaust collection part) is inputted to the model. The air-fuel ratio of each cylinder is estimated by an observer for observing the internal state. The fuel injection amount of each cylinder is corrected according to the deviation between the estimated air-fuel ratio of each cylinder and a target value, thereby making the air-fuel ratio of each cylinder coincide with the target value. In the system, considering that a delay since an exhaust gas exhausted from each cylinder reaches around the air-fuel ratio sensor until the air-fuel ratio of the exhaust gas is detected (hereinbelow, called “response delay of the exhaust system”) changes according to the engine operating state, a map specifying the relation between the response delay of the exhaust system and the engine operating state is created in advance. The timing of sampling an output of the air-fuel ratio sensor (the air-fuel ratio detection timing of each cylinder) is changed with reference to the map in accordance with the engine operating state.
Japanese Patent No. 3217680 also discloses a technique such that, at the time of changing the air-fuel ratio detection timing in accordance with the engine operating state, the air-fuel ratio detection timing is changed in consideration of not only the engine speed, the intake pressure, and the valve timing but also the air-fuel ratio. It describes the relation between response (reaction time) of the air-fuel ratio sensor and the air-fuel ratio as follows. “Since the air-fuel ratio sensor response time becomes shorter when the air-fuel mixture is lean than in the case when the air-fuel mixture is rich, it is preferable to detect the air-fuel ratio at an earlier crank angle (that is, to advance the air-fuel ratio detection timing) when the air-fuel ratio to be detected is lean”. According to a recent study result of the inventors herein, it was found that the change characteristic of the deviation of the air-fuel ratio detection timing according to the air-fuel ratio changes in two opposite ways. If the air-fuel ratio detection timing is advanced when the air-fuel ratio is lean, the air-fuel ratio detection timing is changed in the wrong way. When the air-fuel ratio detection timing of each cylinder is deviated from the proper value, the estimation accuracy of the air-fuel ratio of each cylinder deteriorates, and the state of the cylinder-by-cylinder air-fuel ratio control deteriorates.